Mattress foam?

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My friends and I are converting a school bus in to an RV. I am in charge of all the upholstery and I'm looking for the cheapest, and most comfortable way to make 6 mattress. I don't have exact dimensions yet but I'm thinking they will be around 28' wide and 75' long.

I've looked online and found this site among others for foam. It's the most cost effective so far, but I would love to find something cheaper.
$300 is more than I want to pay!  Do you think it would be even more expensive to go to a craft store?

I will be layering the foam with some thick batting, and I was thinking about making a feather top for each from feather comforters I can get cheap at the Goodwill bins.

I hope to make removable/washable covers too. I'm looking for a material that will keep the foam and feathers from being able to absorb body stuff. What else can I use besides vinyl? I've seen some in stores that have a thin layer of soft material backed with something waterproof. I could try and make something like that.